What would happen to a living creature if it was exposed to
radiation for a long period of time? Would it mutate and die,
mutate and live, its offspring mutate, something else?

John Poletti, a physicist at the National Radiation Laboratory, responded.

Ionizing radiation has three main effects. It can directly damage living
tissue, it can cause cancer to develop later and it can cause mutations in
the offspring (but not the creature exposed).

Direct effects include radiation burns and cataracts in the eyes. There is
a threshold dose below which no direct effect can occur. At very high
doses, damage to the digestive system, destruction of the blood forming
cells in the bone marrow and damage to the central nervous system can be
caused. This can lead to death.

Cancer can be caused by radiation, even at quite low doses, with no
threshold dose. The higher the dose, the higher the chance of cancer being
caused. There is a latent period between exposure to radiation and the
appearance of the cancer, of at least a few years and often twenty years or
more. At low doses, such as for chest x-rays, the chance of cancer can be
neglected in comparison with the normal risks of everyday life. Even at
doses such as those experienced by the Japanese atomic bomb survivors the
radiation cancer risk is still very much less than the natural cancer risk.

Hereditary effects are caused by radiation exposure of the reproductive
organs of either parent (or both). While hereditary effects have been shown
in animal studies, none have yet been observed in humans.

So, what would happen if a creature is exposed to radiation for a long time?
At low dose rates, such as the natural background level, probably nothing
will happen. There is a very small chance of a cancer being caused or of a
mutated offspring. At higher dose rates, the chance of cancer increases as
does the chance of hereditary effects, but no direct effects can occur. At
higher dose rates still, the dose will accumulate sufficiently to exceed the
threshold for direct effects. We would then start to see skin burns or
cataracts. At extreme dose rates, cancer would not be a problem, because
death would occur due to damage to essential bodily systems, long before
cancer could appear!